Free cutting alloy



I Patented May 1 5, 1934 PATENT *OFFICE FREE CUTTING ALLOY Louis .W. Kemp! and Walter A. Dean, Cleveland, Ohio, assignors to Aluminum Company of America, Pittsburgh, Pennsylvania Pa... a corporation of No Drawing. Application September 18, 1933,

Serial No. 689,885

6 Claims.

The invention relates to a method of compounding aluminum base alloys containing substantial amounts of lead, thallium, or bismuth. It relates more specifically to the incorporation of, at least one of these heavier ,elements, in

amount in excess of about 0.5 per cent, in aluminum or its alloys.

. The enumerated elements, comprising those heavier metals in the periodic system having 10 atomic weights between about 204 and 209, have many features in common. Attention is particularly directed to their equilibrium relations with aluminum and their high specific gravity. In the molten condition aluminum and any one of these heavier elements form a two-phase system separated by rather definite boundary surfaces. The solubility of molten lead in molten aluminum, for instance, is very low and until quite recently it had been believed that not more than a small quantity, probably not more than about 0.5 per cent, could be introduced into aluminum base alloys with a uniformity of dispersion satisfactory enough to place the alloy in commercial production. The same is true of thallium and, 5 perhaps to a somewhat lesser extent, of bismuth. These three heavier elements develop similar equilibrium relations with aluminum and they are characterized by specific gravities of the same order, namely, lead about 11.4, thallium about 0 11.9, and bismuth about 9.8. The difliculty in obtaining good liquid miscibility or solubility between aluminum, specific gravity about 2.7, and either lead, bismuth, or thallium can be attributed, in part, to this specific gravity differential. Considerable work has recently been done on the addition of aluminum base alloys of lead, bismuth, and thallium, resulting in the discovery that additions of these heavier elements improve the machining properties of the aluminum alloys 40 to a marked extent. The practical difliculties attendant on the casting of such alloys has contributed to prevent their commercial exploitation. The heavier metal, if present in substantial amounts ormore than about 0.5 per cent, has a 5 tendency to settle to the bottom of the melt. We

have frequently studied this effect by visual examination, by chemical analyses of solidified castings, and by radiographic analysis of the solidified castings. Since for the purposes of the present invention the elements lead, thallium, and bismuth have been found to be substantially similar in their physical characteristics, the invention will be exemplified, for the sake of simplicity, by reference to specific results obtained in adding lead to aluminum and its alloys although the invention also includes within its scope improved methods of adding bismuth and thallium to aluminum and its alloys.

We have come to realize that prior workers have been under a misapprehension with regard 0 to the alloying of lead, bismuth, or thallium with aluminum.v It has heretofore been assumed that when the two metals, in molten condition, were brought into contact by ordinary melting practice, an alloying tendency, if at all present, should immediately make itself apparent. It has been generally assumed that at a temperature clearly above the melting point of both metals, the potentialities of the two elements even for liquid solubility in'each other are soon exhausted and that regardless of the effects of time and temperature, equilibrium conditions are attained very quickly.

We have discovered that two factors combine to produce an important change in the condition of a melt of, for instance, an aluminum-lead alloy. We have discovered that although a lead content of, say, 4 per cent, cannot be held in liquid solubility or homogeneous dispersion in an aluminum alloy when prepared by'the technique ordi- -narily used and completely effective with other alloys, if the temperature be raised to an-extent hereinafter indicated and the melt held at this elevated temperature for a relatively extended period during which it is stirred vigorously, a condition probably of perfect liquid solubility will exist, and, on cooling, a condition of minute and uniform dispersion of lead in the aluminum alloy can be substantially maintained by carefully controlled cooling methods. In the solidified casting made according to the precepts of this invention the lead is uniformly distributed in the aluminum matrix, perhaps partially in sub-microscopic form, partially in very fine particles discernible on microscopic or radiographic examination. Radiographic investigation has led us to suspect that there may be some small amount of lead in solid solution in addition to this minute dispersion. The actual physical condition of the lead is immaterial, for ordinary commercial purposes, provided the dispersion is uniform and the lead particles minute.

The higher the desired lead content, the higher must be the temperature at which the melt is to be held, the longer the time of holding required,

and the more intimate must be the mixing process. In Table I, below, is shown an approximate minimum temperature at or above which a melt of the indicated composition should be held in order to assure complete liquid solubility.

The mobility of the liquid mix increases directly with the temperature. Liquid solution is effected with greater ease as the metal temperature is increased beyond the figure quoted in Table I, but in all cases the temperature prescribed hereinbelow should be substantially. reached or exceeded. Increasing temperatures tend to shorten the required time correspondingly.

The time of exposure to the temperatures noted above can be indicated herein closely enough for practical effectiveness, although, except for economic reasons, there can be no disadvantage to holding the melt at temperature for longer than the periods given. The two liquid phases, molten lead and molten aluminum, which under ordinary conditions would solidify separately, should be thoroughly stirred while the melt is held at temperature to assist the process of solution and this fact should be borne in mind in following the directions herein. With lead contents of 5 per cent or less the time at heat need not exceed 30 minutes, the time varying directly as the com position. As the lead content is raised from 5 per cent to 20 per cent, the time should be proportionally increased from 30 minutes to about arli1 lliour, the melt being vigorously stirred meanw 1 e.

When the aluminum-lead melt has been reduced to a single liquid phase as explained hereinabove, the melt should be cast under conditions assuring a rapid cooling from the liquid solution zone to the freezing point of the particular alloy. Castings are ordinarily made either in molds of agglomerate mixtures such as sand, or in permanent molds wherein either the shape of the final object is immediately defined or, as in ingot cast ing, an article is produced which is subsequently worked down to final form by mechanical deformation.

The specific directions given hereinaboveare directly applicable to aluminum-lead alloys but thallium or bismuth may be substituted for the lead in all instances without substantial variation the molten head of metal in the temperatures or times stated.

Early efforts to cast aluminum-lead alloy ingots containing more than about 0.5 per cent of lead resulted in fatally injurious segregation of lead content at the bottom of the ingot mold. We have found that we can producean ingot in which a substantial amount of lead is uniformly dispersed in minute form by heating the metal and maintaining the temperature as discussed hereinabove, and then, as a final step, controlling the solidification so that the greater part of in the ingot mold is at or above the solution temperature for the particularcomposition, while the actual solidification zone is artificially restricted to a narrow plane. To effect this purpose, we use to a substantial extent the method disclosed in U. S. Patent 1,777,657 to Stay and Holzhauer and in addition we maintain the molten head of metal 'at or above the temperatures given in Table I for the particular composition which is being cast. The method disclosed in the aforesaid patent comprises causing an ingot to solidify from the bottom upward in a gradually rising plane of solidification and a commercial embodiment thereof consists in immersing a mold filled with molten metal in a bath of cooling liquid thereby causing a zone of sharp temperature differential to travel from the mold bottom upwardly toward the top. In a practical case we have causeda water spray to be moved upwardly along the outer mold surface while the molten metal within the mold above the solidification zone is maintained at a desirably high temperature by the previously heated condition of the mold, assisted by the application of a heating agent, such as a gas burner arrangement, immediately above the plane of the water spray.

In a related method, also effective in the commercial application of our invention, we pour the molten metal, at a temperature such as indicated in Table I, for the melt compositions, through a pipe extending downward'into the mold which is completely submerged in water to within a few inches of its top. The bottom of the pipe is provided laterally with narrow slits through which the metal is sprayed in a horizontal plane producing substantially an immediate solidification zone which gradually rises as the pipe is elevated, keeping the bottomof the pipe just above the ascending solidification zone. It is apparent that by this and the preceding method the transition from the liquid solution, single phase condition to the solidified ingot, is so rapid that the result of the settling out process is an ingot in which the lead content is either submicroscopically minute, or, even though discern- .ible, is so uniformly refined that a thoroughly satisfactory alloy condition prevails. The examples given hereinabove illustrate the significance of the term "controlled cooling as used herein in describing one of the necessary adjuncts of the invention.

Ingots of aluminum-bismuth and aluminumthallium alloys may be made up in the manner herein disclosed with relation to aluminum-lead alloys.

Ingots of aluminum-base alloy vary considerably in size from the smallest, perhaps 3 or 4 inches on a side and 10 inches long, tothe largest (at present), 20 inches on a side and more than 4 feet long. The measures used in'controlling the cooling in the large sizes do not differ in theory from those used in the smallest sizes, but the chilling technique must be more severe as the section thickness increases.

In making castings to be used as, such (i. e.,-

without mechanical deformation), the section thickness involved is practically always considerably less than that of an ingot and as a consequence extreme chilling measures are not required in order to prevent settling out. Chill mold castings, frequently known as permanent mold castings, produced in molds which abstract.

whereby freezing may be considerably accelerated.

When other elements in addition to lead, hismuth or thallium are added to an aluminum melt, they alter to some extent the amount of the said heavy element which can be incorporated as a liquid solution at any given temperature and as a consequence the amount which can be uniformly dispersed in the solidified casting. As an instance of this effect, we have determined by actual tests that at a temperature of about 1475 F. about 4.20 per cent lead can be carried in liquid solution in molten aluminum, but if to this melt there is added about 4 per cent copper, the lead content in liquid solution at the same temperature is reduced to about 3-85 per cent.

In the accompanying claims, the term aluminum-base alloy signifies an alloy containing more than 50 per cent of aluminum, and the term alloy signifies that the lead or other alloying elements added to the aluminum are present in sufficiently uniform dispersion to satisfy commercial requirements.

We claim:

1. A method of making an aluminum base alloy containing more than 0.5 per cent of at least one of the heavy elements lead, bismuth or thallium, the method comprising heating a molten mix containing more than 0.5 per cent of the heavy element to a temperature at which its homogeneous liquid solubility in the aluminum is obtained, maintaining the solution tem perature for a period required to effect said homogeneous liquid solution while stirring vigorously, and finally effecting a controlled and rapid drop in temperature of the melt from said temperature of homogeneous liquid solution to the freezing point of the melt.

2. A method of making an aluminum base al- 10y containing from 0.5 per cent to 20.0 per cent lead, the method comprising heating a melt of the aforesaid lead content to a temperature of between 1250 F. and 1750 F., holding the melt within said temperature range for from 10 minutes to 60 minutes while maintaining a vigorous and intimate admixture of the liquid components, and finally cooling the melt rapidly enough to its freezing point to'maintain a uniform distribution of the lead constituent in the aluminum.

3. A method of uniformly incorporating more than 0.5 per cent of at least one of the heavier elements, lead, bismuth or thallium, in an aluminumbase ingot for working, the method comprising heating a molten mix containing more than 0.5 per cent of at least one of said heavier elements to a temperature at which homogeneous liquid solubility in the aluminum is effected, maintaining the solution temperature for a period .required to effect said homogeneous liquid solubility while stirring vigorously, and finally causing said melt to solidify in a suitable ingot mold by rapidly chilling the metal in a rising plane of solidification while maintaining that portion of the molten metal above the solidification zone at the temperature required to preserve the condition of homogeneous liquid solution.

4. A method of uniformly incorporating more than 0.5 per cent of lead in an aluminum-base ingot for working, the method comprising heating a molten mix of aluminum-base alloy containing more than 0.5 per cent lead to a temperature at which homogeneous liquid solubility of the lead constituent in the aluminum is obtained, maintaining the solution temperature for a period required to efiect said homogeneous liquid solubility while stirring vigorously and finally causing said melt to solidify in a suitable ingot mold by pouring the homogeneous molten solution through a pouring tube, perforated at its lower end, into an ingot mold, the temperature of said ingot mold being maintained at a point substantially lower than the temperature of the molten solution, so as to cause the metal to solidify practically immediately in a gradually rising plane of solidification.

5. A method of uniformly incorporating more than 0.5 per cent of bismuth in an aluminumbase ingot for working, the method comprising heating a molten mix of aluminum-base alloy containing more than 0.5 per cent bismuth to a temperature at which homogeneous liquid solubility of the bismuth constituent in the aluminum is obtained, maintaining the solution temperature for a period required to effect said homogeneous liquid solubility while stirring vigorously and finally causing said melt to solidify in a suitable ingot mold by pouring the homogeneous molten solution through a pouring tube, perforated at its lower end, into an ingot mold, the temperature of said ingotmold being maintained at a point substantially lower than the temperature of the molten solution, so as to cause the metal to solidify practically immediately in a gradually rising plane of solidification.

6. A method of uniformly incorporating more than 0.5 per cent of thallium in an aluminumbase ingot for working, the method comprising heating a molten mix of aluminum-base alloy containingmore than 0.5 per cent thallium to a temperature at which homogeneous liquid solubility of the thallium constituent in the aluminurnis obtained, maintaining the solution temperature for a period required to effect said homogeneous liquid solubility while stirring vigorously and finally causing said melt to solidify in a suitable ingot mold by pouring the homogeneous molten solution through a'pouring tube, perforated at its lower end, into an ingot mold, the temperature of said ingot mold being maintained at a point substantially lower than the temperature of the molten solution, so as to cause the metal to solidify practically immediately in a gradually rising plane of solidification.

LOUIS W. KEMPF. WALTER A. DEAN. 

